This invention relates to the processing of nickel-iron-base alloys, and, more particularly, to the refinement of ingot grains through the precipitation of delta-phase precipitates.
A semifinished wrought nickel-base alloy article is conventionally made by first melting blends of the constituent elements of the alloy. The final cast ingot is produced using one or more cast and remelt steps. Primary thermomechanical working of the ingot (termed "ingot conversion") converts the structure of the cast ingot from a coarse as-cast dendritic grain structure to a fine equiaxed billet grain structure (termed a "cast-wrought" structure). The primary conversion step in highly alloyed metals such as superalloys also serves to break up interdendritic regions, thereby reducing the segregation that occurs during solidification of the ingot. The semifinished billet is then further processed into its final geometry by forging, heat treating, and machining. An example of a nickel-iron-base alloy manufactured into articles by this approach is alloy 718.
Using conventional casting methods the average dendrite grain size in as-cast ingots is typically coarser than ASTM 1 and often coarser than ASTM 00 grain size. If such a material were processed directly into the shape of the final product, an insufficiently high level of strain would be introduced to allow uniform recrystallization of the ingot structure. The result would be non-recrystallized regions that retain the large as-cast grain structure of the ingot. Because grain size is one of the primary parameters controlling properties in these materials, the non-recrystallized regions will typically either fail the microstructural requirements for the final product or, if not detected, will lead to accelerated mechanical failure of the product. The thermomechanical ingot conversion of the ingot into billet prevents the presence of as-cast grains in the billet and also mitigates associated chemical segregation in the billet, ensuring a fine-grain structure in the billet.
Conventional ingot conversion processing is an expensive operation. The function of the primary thermomechanical working is to recrystallize the material using the strain added during each working operation to nucleate new grains at existing grain boundaries. Commercial ingots of nickel-iron-base superalloys often weigh thousands of pounds. Because of the large size of these ingots, it is difficult to achieve a uniformly high level of strain for recrystallization in a single thermomechanical operation. The large-grain areas in the ingot, having a lower grain boundary density, require more cumulative strain to achieve a uniform final grain size. The ingot conversion therefore typically requires multiple upset and draw operations. In theory, the coarser the starting grain size the more thermomechanical operations will be required to refine the grain size.
The equipment to accomplish the multistep thermomechanical conversion in commercial practice is large in size and expensive. The rough thermomechanical working also requires skilled operators, and careful control over the processing practices. This equipment requirement and extensive processing cannot be avoided or significantly reduced in scale under conventional practice because of the non-uniformity in as-cast grain structure due to differences in solidification rates across the large cross-section of the large ingots.
There is a need for an improved approach to the mechanical processing of ingots into final articles, which reduces the required processing and still results in an article of acceptable, or even improved, final quality. The present invention fulfills this need, and further provides related advantages.